1. Field of the Invention
This invention relates to the characterization of magnetic materials and, in particular, to a method and apparatus for measuring certain parameters of an epitaxial magnetic garnet films.
More specifically, this invention relates to a determination of the stripe width and collapse field so that taken together with the film thickness, the properties of the magnetic material can be deduced. This is particularly useful in characterizing magnetic films for magnetic domain memory chips.
2. Description of the Prior Art
The characterization of static magnetic properties of bubble materials of a known composition can be made by determining three parameters of the material; the film thickness, the stripe width and the collapse field. With these three parameters, calculations can be made of the film magnetization, anisotropy, exchange constant, Neel temperature and the Q of the film. There are two main prior art techniques used to determine the stripe width and collapse field. One technique is to use polarization microscopy to directly measure the stripe widths and then apply a magnetic field to the garnet film to see at what field a magnetic bubble collapses. This method is direct but it is also time consuming and, thus, uneconomical. Also, polarization microscopy becomes less feasible as bubble diameters approach submicron dimensions because the diameters of collapsing bubbles approaches the diffraction limit of the microscope. Also, as bubble domains become smaller, the film becomes correspondingly thinner and the Faraday rotation and the resulting contrast decrease accordingly.
Another technique for measuring the stripe width and collapse field is a so-called "spatial filtering" technique and is described by R. D. Henry in an article, Bubble Material Characterization Using Spatial Filtering, Material Research Bulletin, Vol. II, pp. 1285-1294, 1976, Pergamon Press, Inc. In this technique, a polarized laser beam is directed thru a film. The film has a random orientation of the stripe domains. A diffraction pattern of the randomly oriented domains produces a conical type diffraction pattern. Then, at bias field (Ha=0) a spatial filter of an annular pass-ring configuration is moved back and forth to obtain maximum intensity of the first order diffracted light which is focused by a lens onto a photomultiplier or detector. The photomultiplier or detector produces a current as a function of light intensity. The first order diffraction angle is used to obtain the stripe width, then, the spatial filter is moved a calculated distance toward the film so that the spatial filter will pass only the second order diffraction beam. Then Ha is adjusted to maximize the current detected by the photomultiplier. The collapse field is calculated from this value of current and the stripe width.
The disadvantages to this latter prior art method are, first, one needs the sliding spatial filter to eliminate stray light and to maximize the signal of the diffracted beams and, second, calculations are necessary to determine where to locate the spatial filter for the second order diffraction test--all of which are time consuming even with the benefit of a calculator. Finally, a condensing lens is required to focus the diffracted beam onto the detector since detectors usually aren't large enough to absorb the total diffracted beam.
Accordingly, it is an object of this invention to provide a method and apparatus for characterizing magnetic material which overcomes the deficiencies of the prior art system by eliminating the spatial filter, all mechanical feedback and mechanical adjustments.